1. Field of the Invention
This invention relates to cellular communications, and in particular, it relates to an FFR-based network MIMO transmission architecture in multi-cell wireless OFDM/OFDMA systems.
2. Description of the Related Art
Network multiple-input multiple-output (MIMO), which has been proposed for potential deployment in fourth generation (4G) wireless system, is a technique aimed to mitigate inter-cell interference and enhance throughput by coordinating multi-cell transmission among a number of geographically separated antennas belongs to the same or different base stations (BS). This kind of multi-BS MIMO techniques improves sector throughput and cell-edge throughput through multi-BS cooperative signaling. Multi-BS joint MIMO processing may be enabled by the BS for one or several mobile stations (MS) when joint precoding is applied in the serving and neighboring cells. Take the example from the wireless broadband system Worldwide Interoperability for Microwave Access (WiMAX), there are two scenarios: closed-loop macro diversity (CL-MD) and collaborative MIMO (Co-MIMO) transmission. For CL-MD transmission, a single MS is served jointly by multiple coordinating BSs. For Co-MIMO transmission, several mobile stations (MS) are served jointly by the multiple coordinating BSs through MU-MIMO scheduling and precoding. Similar concept for multi-BS joint MIMO processing is also proposed in Third Generation Partnership Project (3GPP) Long-Term Evolution-advanced (LTE-A) standard, referred to as coordinated multi-point (CoMP) transmission. There are also two scenarios: joint processing (JP) and coordinated scheduling/coordinated beamforming (CS/CB). Both Co-MIMO and CoMP-JP transmissions (often referred to as network MIMO in research papers) allow multiple coordinated BSs to serve multiple users simultaneously with jointly designed precoding.
A reference that describes network MIMO is M. K. Karakayali, G. J. Foschini, and R. A. Valenzuela, “Network coordination for spectrally efficient communications in cellular systems,” IEEE Trans. on Wireless Commun., vol. 13, no. 4, pp. 56-61, August 2006 (“Karakayali et al. 2006”). This reference describes “network coordination as a means to provide spectrally efficient communications in cellular downlink systems. When network coordination is employed, all base antennas act together as a single network antenna array, and each mobile may receive useful signals from nearby base stations.” (Id., Abstract.) In this reference, “the objective is to coordinate the base antenna transmissions so that the signals from multiple base antennas can be coherently received to improve signal quality. Among the transmission techniques achieving this objective, two simple forms of coherent coordination are considered . . . : coordination by zero-forcing transmission and coordination by dirty paper coding combined with a limited form of zero forcing.” (Id., pages 56-57.)
Several other general approaches are used to mitigate inter-cell interference in conventional cellular systems, including frequency reuse, cell sectoring and spread spectrum. One commonly used technique is to avoid using the same set of frequencies in neighboring cells; that is, a cluster of cells share the entire transmission spectrum. This approach leads to the decrease of the number of available channels within each cell.
A scheme known as fractional frequency reuse (FFR) has been proposed to improve spectrum efficiency by applying the reuse partition technique. FFR, also called frequency partition or reuse partition, allows different frequency reuse factors to be applied over different frequency partitions during the designed period for transmission. FIG. 9 shows a conventional FFR scheme for omni-cells. In this and some other illustrations in this disclosure, the cell shape is shown as hexagons (sometimes also referred to as diamond-shaped cells). The entire useable frequency range is partitioned into an inner frequency band fA for use in inner cell regions (cell center) of all cells, and an outer frequency band fB for used in outer cell regions (cell edge). The frequency band fB is further partitioned into three subbands fB1, fB2, and fB3; as shown in FIG. 9, adjoining (neighboring) cells use different subbands in the outer cell regions. In the illustrated example, adjoining cells 0, 6 and 5 use subbands fB1, fB2, and fB3, respectively for their outer cell regions. Only one of the three subbands fB1, fB2, and fB3 is used by each cell (e.g. cell 0 does not use subbands fB2 and fB3). In such an FFR scheme, signal quality at cell edge is improved at the cost of lower spectrum usage.
To improve spectrum efficiency, FFR can be applied to a sectorized cellular system as shown in FIG. 10. A sectorized cellular system is one in which each cell is divided into multiple sectors by using directional antennas. In the illustrated example, each cell is divided into an inner sector and three outer sectors Sector 1, Sector 2, and Sector 3. The frequency spectrum is divided in the same way as in FIG. 9, i.e., into frequency bands/subbands fA, fB1, fB2, and fB3. As shown in FIG. 10, each cell uses the inner frequency band fA for its inner sector, and uses the three frequency subbands fB1, fB2, and fB3 for its three outer sectors, respectively. The correspondence between the three subbands and the three sectors 1, 2, 3 is identical among all cells. As a result, in an area where three sectors from three different cells adjoin each other (e.g., Sector 3 of Cell 0, Sector 2 of Cell 6 and Sector 1 of Cell 5), the frequency subbands used by the three sectors are all different, which reduces inter-cell interference. In such an FFR scheme, each cell can transmit using the entire frequency spectrum.
FIGS. 9 and 10 are adopted from Farooq Khan, “LTE for 4G mobile broadband: air interface technologies and performance,” Cambridge University Press 2009, pp. 419-420.